The present invention relates to cathode ray display tubes which exhibit high contrast and are thus readable by a viewer with high ambient light levels, such as bright sunlight. The brightness of the phosphor screen does affect the viewing under such conditions, but even more important is the contrast between the activated phosphor areas and the reflected ambient light from the unactivated phosphor areas which determines the readability of the display.
Numerous techniques have been practiced to reduce the light reflection from the tube faceplate and to thereby improve contrast. These include the use of anti-reflective coatings on the outer faceplate surfaces, and the use of dark, low neutral density transmissivity glass as the faceplate to reduce the reflected light level. Recently, color selective laminated panels whose peak transmission color coincides with the emission color of the phosphor screen have been placed on the exterior of the tube faceplate. The use of such laminated color selective panels adds to the manufacturing costs of the tube.
The contrast ratio of a cathode ray display tube is the luminance of the displayed information divided by the luminance (B) of the area immediately surrounding it. The luminance of the displayed information is the sum of the phosphor excitation luminance (S) and the surrounding area luminance (B). The contrast ratio is then: EQU C=S+B/B or C=1+S/B.
In some applications a factor termed contrast is defined as C-20 log (1+S/B). The contrast ratio or contrast of a tube is determined by measuring the luminance (S) in subdued light with the tube operating at some specified level. The luminance (B) is determined with the tube off by measuring its luminance caused by diffuse reflection of light of a specified intensity incident at a specified angle to avoid any contribution from specular reflection.
The advantage of a filter glass is that it has a relatively high transmission of the phosphor light compared to the incident ambient white light. Thus, for a given tube operating level and phosphor input energy a higher (S) is obtained or conversely for a given (S) less input energy to the phosphor is required, resulting in longer tube life and performance.